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Thursday, March 15, 2012

Big Engineering 49 - Twice an Hour to Orbit

   Joseph Friedlander has an article on Next Big Future which gives all sorts of technical detail about how we could build an orbital launcher that could reach and return from orbit twice an hour.

  This amounts to a vessel loading pure water and carrying a reactor which will superheat the water to produce a steam rocket and take the craft to the edge of space where it releases a cargo rocket. At that height with no air friction even an amateur rocket could reach orbital velocity.

  Then it comes back, lands in the water, reloads with cargo and water and does it again. Twice an hour.

   This is how he puts it with the technological stuff shorn off.

Powered by a reactor light enough to fly, hot enough and with power enough to energize a reaction fluid (hopefully a light fluid with high exhaust velocity) and efficient enough to carry a large fraction of launch weight as payload to low orbit.

So a robust and buildable nuclear booster, massively reusable (a couple times an hour not a week) that uses fresh water as reaction mass (cheapest fluid available) that takes off and lands (and is based) in freshwater and repumps itself full within minutes for another run would be capable of greatly efficient amortization of costs and great economies of use.

It would accelerate for under 90 seconds, hop up to space and eject the multi-staged chemical rocket from an internal bay—the “garage”—the equivalent of an air launch but in space.

We have just now described a ’hopper’ architecture for a nuclear steam rocket. What is the difference between a nuclear steam rocket and a conventional steam rocket--such as the rocket used by Evel Knievel in his attempt to jump the Snake River Canyon near Twin Falls, Idaho on September 8, 1974.
A conventional steam rocket uses preheated water on the ground for launch with a simple pressure valve. Alas, the exhaust velocity is a mere 300 m/s or so.

What is different here? A nuclear reactor is heating the water literally on the fly—so we are not storing superpressured steam or supercritical water but low-pressure water in light tanks and heating it live. Exhaust velocity is around 6 times greater.

   This does not in any way expose radioactives to the atmosphere. The steam no more contacts the core than it does in a pressurised water reactor. They do release heated water into the sea as this releases steam into the air. In fact it very much the same thing. Or if you prefer similar to atomic powered ships of which there have been many, from commercial vessels to icebreakers through to aircraft carriers. It is hardly possible to sensibly argue against this craft unless they have been shown to be a problem.

   I assume Joe Insists on fresh water because salt water would be more corrosive. Otherwise these could be sea launched from anywhere in the equatorial zone. However, serendipitously, one of the products of the OTEC power systems suggested by Marshall Savage to power floating seasteds is distilled water.

   Because reactors have a minimum practical size and can become more powerful faster than they go up in weight this system is highly scalable.

For a 180 gigawatt ship (12000 ton class liftoff weight)—Super Saturn V class cycling twice an hour for a year that is .....8081.1 kilograms of U-235 per year (at $100000 a kilogram, over $800 million worth that must be replaced)...

for space boost to greatly lower costs of launching—it sounds nearly unbeatable. Rather than something like 1.3 billion to send a 50 ton spacecraft to the Moon, we can imagine doing it for something like $10 million (exclusive of the spacecraft of course). 2000 tons starting at 100 km even at standstill could send that much to the Moon in perhaps 4-5 stages. Doing that a few thousand times a year would enable the beginnings of Lunar colonization. Something to think about.

  With small modular reactors being about to be built at under £200 million each such a craft should be "easily within the means of even the smallest nuclear power." While this does not inherently require technology only available to government, as the Orion nuclear explosion powered ship did, it could, in theory, be done purely by private enterprise. In practice I think it would require the explicit support of government, though not necessarily, its money.

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